In recent years, pressure type flow control devices have been widely adopted in place of mass flow controllers, for example, in semiconductor manufacturing equipment and chemical manufacturing equipment. Pressure type flow control devices generally adopt a so-called “metal diaphragm type” control valve due to its advantages of higher corrosion resistance, lower dust generation, better gas replacement, and higher opening and closing speed. Also, pressure type flow control devices generally adopt a piezoelectric driven type drive unit (i.e., a piezoelectric actuator) due to its advantages of larger thrust, better responsiveness and controllability.
The structure of metal diaphragm type control valves, in which a piezoelectric actuator (piezo-positioner) is used for the drive unit, are conventionally known as disclosed in Japanese Published Unexamined Patent Application No. 2003-120832 (Patent Document 1), etc., for example.
That is, as shown in FIG. 11, such a metal diaphragm type control valve 30 is formed as a normal close-type piezoelectric driven control valve including: a body 31 with a valve chamber 31a and a valve seat 31b formed therein; a metal diaphragm 32 disposed inside the valve chamber 31a to be in contact with, and separated from, the valve seat 31b, a holding adapter 33 for pressing the outer peripheral edge of the metal diaphragm 32 in an air tight manner toward the body 31; a half-split base 34 for pressing the holding adapter 33 toward the body 31; a base presser 35 for fixing the holding adapter 33 and the split base 34 to the body 31; an actuator box 36 supported ascendably and descendably by the base presser 35; a diaphragm presser 37 inserted and installed at the lower end of the actuator box 36 so as to be in contact with the metal diaphragm 32; a disc spring 38 provided between the split base 34 and the actuator box 36 so as to press and urge the actuator box 36 downward; a piezoelectric actuator (i.e., a piezo-positioner) 40 housed inside the actuator box 36 with the lower end thereof supported on the split base 34 via a ball receiver 39; and an adjustment cap nut 43 threadably mounted on the upper end portion of the actuator box 36 so as to positionably support the upper end of the piezoelectric actuator 40 via a bearing receiver 41 and a bearing 42, wherein extension of the piezoelectric actuator 40 due to application of a voltage causes the actuator box 36 to ascend while supported by the base presser 35 against the elastic force of the disc spring 38 so as to, accordingly, cause the metal diaphragm 32 to be separated by the elastic force thereof from the valve seat 31b and the valve is opened. On the other hand, the removal of voltage applied to the piezoelectric actuator 40 causes the piezoelectric actuator 40 to return to its original length dimension from an extended state and, at the same time, the actuator box 36 is pressed down by the elastic force of the disc spring 38 to accordingly cause the metal diaphragm 32 to be pressed downward by the diaphragm presser 37 so as to come into contact with the valve seat 31b and, thereby, the valve is closed.
The piezoelectric driven control valve 30, in which the axial center of all the members can be aligned automatically, has the advantage that the assembling accuracy increases significantly so that variation in assembling accuracy, and the hysteresis phenomenon observed regarding valve stroke during opening and closing operations becomes less likely to occur.
The piezoelectric actuator (piezo-positioner) 40 used in the piezoelectric driven control valve 30 is a stacked-type piezo-positioner in which stacked-type piezoelectric elements 40b are housed in a sealed manner inside a metal casing 40a as shown in FIG. 12. This stacked-type piezoelectric actuator 40 is arranged in a manner so that with extension and retraction of the piezoelectric elements 40b, a hemispherical displacement member 40c, provided at the leading end of the casing 40a, reciprocates along the axial center of the piezoelectric actuator 40.
However, the stacked-type piezoelectric actuator 40 suffers from a problem caused by the fact that the extension and retraction rate of the piezoelectric elements 40b is greater than that of the displacement member 40c when the piezoelectric actuator 40 is extended and retracted, which causes a tensional force to be applied between piezoelectric elements 40b adjacent each other when the piezoelectric actuator 40 is retracted, which causes damage to the piezoelectric elements 40b and shortens the product lifetime. To address this problem, it is necessary, when actually operating the stacked-type piezoelectric actuator 40, to apply a precompression load to the piezoelectric elements 40b in the retraction direction because the piezoelectric elements 40b have a low tolerance for tensional force. The application of the precompression load in the retraction direction relieves the tensional force applied between piezoelectric elements 40b adjacent each other to prevent damage to the piezoelectric elements 40b from the tensional force.
In accordance with the conventional piezoelectric driven control valve 30 shown in FIG. 11, because the actuator box 36 is pressed and urged downward by the elastic force of the disc spring 38, the piezoelectric actuator 40 is in a state precompressed by the elastic force of the disc spring 38. However, the disc spring 38 used in the piezoelectric driven control valve 30 is provided to press and urge the actuator box 36 downward so as to cause the diaphragm presser 37 to bring the metal diaphragm 32 into contact with the valve seat 31b. Therefore, if the disc spring 38 has a very strong elastic force, then the metal diaphragm 32 and/or the valve seat 31b may be damaged. For this reason, the disc spring 38 in the piezoelectric driven control valve 30 cannot have a very strong elastic force. Accordingly, the piezoelectric actuator 40 also cannot be applied with a very large precompression load because, otherwise, it may suffer from the problem wherein the piezoelectric elements 40b are damaged by application of a large tensile stress.
The conventional piezoelectric driven control valve 30 suffers from another problem in that when it is used under a high-temperature environment, such as at a temperature of 100 degrees C. or more, a gap forms between the upper end portion of the piezoelectric actuator 40 and the adjustment cap nut 43 threadably mounted on the upper end portion of the actuator box 36 due to thermal expansion of the actuator box 36 that houses the piezoelectric actuator 40. This gap causes a generation force, when the piezoelectric actuator 40 is extended, to not be transmitted reliably and successfully to the actuator box 36, which makes high-precision flow control difficult. In particular, because the amount of displacement of the piezoelectric actuator 40 is extremely small, even a slight amount of thermal expansion of each member (e.g., actuator box 36) in the control valve 30 will have a great impact on the flow control characteristics of the conventional piezoelectric driven control valve 30. To solve this problem (namely, the gap formation due to thermal expansion of the actuator box 36), it may be useful to make a control valve 30 that has a structure in which the piezoelectric actuator 40 is applied preliminarily with an external compression force of approximately 200N. However, such a control valve has not yet been developed.
Patent Document 1: Japanese Published Unexamined Patent Application No. 2003-120832